1. Field of Invention
The present invention relates to a method of adjusting a scanning exposure apparatus for transferring a pattern formed on a mask onto a photosensitive substrate during a photolithography process. The invention also relates to a scanning exposure apparatus using the above adjustment method. More particularly, the invention relates to a method suitable for adjusting the image focusing characteristics of the projecting optical system of the scanning exposure apparatus.
2. Description of Related Art
A one-shot type projection exposure apparatus (stepper using the step-and-repeat process) is widely used to produce a semiconductor device or the like. With this apparatus, an image of a pattern formed on a reticle serving as a mask is transferred, via a projecting optical system onto a photosensitive substrate. With the expansion of the pattern area of semiconductor chips, it becomes necessary to further expand the exposure area of the projection exposure apparatus used to produce these chips. However, a technical difficulty arises in simply expanding the exposure area while trying to keep good image focusing characteristics of the projecting optical system. Even if the expansion is possible, the resultant projection exposure apparatus would have a great size and would be prohibitively expensive.
One know technique of effectively expanding the exposure area without expanding the effective physical exposure field of the projecting optical system is to employ a step-and-scan projection exposure apparatus. In this technique, after stepping a wafer to a scanning start position, the wafer and a reticle are scanned synchronously relative to a projecting optical system. The reticle is illuminated by exposure light via a slit with a long and narrow rectangular shape or a circular arc shape. Thereby, the image of a reticle pattern is transferred onto a wafer from one shot area to another.
In the case of a scanning projection exposure apparatus, such as a step-and-scan exposure apparatus, the size of a circuit pattern formed on a wafer along the scanning direction is not limited by the size of the projecting optical system. On the other hand, the maximum pattern size (an area in which an image can be formed) in the direction perpendicular to the scanning direction is nearly equal to the diameter of the effective exposure field of the projecting optical system. Therefore, unlike the conventional one-shot projection exposure apparatus in which the available area is limited to a square or rectangle inscribed in the effective field of its projecting optical system, the scanning exposure apparatus has the ability of exposing a wider area to form a pattern.
Furthermore, in the scanning exposure apparatus, only a part of the effective exposure field of the projecting optical system is used. That is, in the slit-shaped exposure area, the image focusing characteristics, such as the distortion and the magnification error, of the projecting optical system can be kept within allowable ranges. As a result, it is possible to achieve high-accuracy image focusing characteristics associated with the transferred image over the entire shot area. In the conventional technique, when a scanning exposure apparatus is assembled and adjusted, a test pattern image is projected at each measurement point within the slit-shaped exposure area of the projecting optical system and the positions and contrast of the projected images are measured. In accordance with the result of the measurement, the image focusing characteristics of the projecting optical system are adjusted.
In the conventional scanning exposure apparatus, among various image focusing characteristics of the projecting optical system, random components independent of the distance from the optical axis are easy to adjust since the adjustment can be performed within a narrow area. On the other hand, for systematically varying components of the image focusing characteristics, such as those varying in proportion to the distance from the optical axis, the narrow measurement area results in less amount of information available for use in the adjustment. This makes it difficult for the scanning exposure apparatus to make a high-precision adjustment compared to the one-shot type (step-and-repeat) projection exposure apparatus.
Until recently, adjustment errors which occur in the conventional techniques were within allowable ranges. However, as the size of semiconductor devices becomes increasingly smaller, the image focusing characteristics of the projecting optical system become more critical. Thus, the conventional techniques of adjusting the image focusing characteristics of the projecting optical system have difficulty in achieving required characteristics.